The present invention generally relates to semiconductor lasers and methods of producing the same, and more particularly to a semiconductor laser which has a double heterostructure and is designed to produce a large output and to a method of producing such a semiconductor laser.
Presently, as one means for increasing the memory capacity of computer systems, there are proposals to use an optical disk unit or a magneto-optic disk unit as a memory device. Accordingly, there is a demand for a semiconductor laser which can produce a large output so that the semiconductor laser may be used as a light source when storing information on the disk unit with a high density. For example, a visible-light semiconductor laser which emits a laser beam having a wavelength in the order of 0.6 .mu.m is suited as the above light source.
Generally, AlGaInP system semiconductor lasers are made using metal organic vapor phase epitaxy (MOVPE). The reason for using the MOCVD is that the segregation coefficient of Al is too large and it becomes impossible to control the Al content when an attempt is made to grow the AlGaInP system semiconductor crystal by liquid phase epitaxy (LPE).
Various structures for controlling the mode of the semiconductor laser have been proposed and reduced to practice. However, most of the proposed structures are made using the LPE and utilize the growth peculiarity of the LPE such as the anisotropy. Very few of the proposed structures use the peculiarity of the MOVPE. In addition, even when an attempt is made to make using the MOVPE the semiconductor laser having the structure which is intended to be made by the LPE, it is extremely difficult to make the semiconductor laser by the MOVPE due to the growth peculiarity of the MOVPE.
For the above described reasons, there is a need to realize a semiconductor laser structure which is suited to be made utilizing the growth peculiarity of the MOVPE. Particularly, the realization of such a semiconductor laser structure is essential in the AlGaInP system semiconductor lasers.
The mode control structure of the known semiconductor lasers can roughly be divided into three kinds.
According to a first kind of mode control structure, a substrate side cladding layer, an active layer and a surface side cladding layer are successively formed on a flat substrate, and a current confinement structure for achieving lateral confinement of the current path or a ridge structure is formed at a part above the active layer. In this specification, this first kind of mode control structure will be referred to as the ridge type structure.
On the other hand, according to a second kind of mode control structure, a substrate side cladding layer, an active layer and a surface side cladding layer are successively formed on a flat substrate. Then, a mesa etching is made to a part which is deeper than the active layer, and a buried layer is formed to bury the mesa structure. In this specification, this second kind of mode control structure will be referred to as the buried type structure.
Furthermore, according to a third kind of mode control structure, a substrate is subjected to a predetermined process to make the substrate surface non-uniform, and a substrate side cladding layer, an active layer and a surface side cladding layer are successively formed on the non-uniform substrate surface. In this specification, this third kind of mode control structure will be referred to as the shaped substrate type structure.
In the following description, the index of crystal face and the stripe direction are defined as follows. That is, it is known that from the crystal point of view, there exist a plurality of faces which are equivalent to the (100) face. For example, the (001) face is equivalent to the (100) face, and in this case, the &lt;110&gt; and &lt;110&gt; directions are respectively equivalent to the &lt;011&gt; and &lt;011&gt; directions. It is confusing to list all the equivalent faces, and in this specification, the (100) face represents a plurality of faces which are equivalent to the (100) face, and the stripe directions which correspond to the &lt;011&gt; and &lt;011&gt; stripe directions are respectively represented by the &lt;100&gt; and &lt;110&gt; directions. In addition, the faces which are equivalent to the (111) face which appears at the sloping surface of the conventional mesa when the conventional mesa stripe is formed on the (100) face in the &lt;011&gt; direction will be represented by the (111)B face. The faces which are equivalent to the (111) face which appears at the sloping surface of the conventional mesa when the conventional mesa stripe is formed on the (100) face in the &lt;011&gt; direction will be represented by the (111)A face.
FIG. 1 shows a cross section of an essential part of a conventional semiconductor laser having the ridge type structure. The semiconductor laser shown in FIG. 1 includes an n-type GaAs substrate 1, an n-type AlGaInP cladding layer 2, an InGaP active layer 3, a p-type AlGaInP cladding layer 4, a p-type InGaP buffer layer 5, an n-type GaAs current confinement layer 6, a p-type GaAs cap layer 7, a p side electrode 8 and an n side electrode 9.
This ridge type structure is popularly used in the AlGaInP system semiconductor lasers for the following reasons. That is, because the n-type cladding layer 2 to the p-type cladding layer 5 can be successively formed on the substrate 1 in one growth process, there is only a small possibility of a defect being formed in the active layer 3. Accordingly, it is possible to form a satisfactory double heterostructure. In addition, although it is difficult to obtain a satisfactory surface morphology in the case of a layer including Al, there is no need to form an AlGaInP layer on the layer which includes Al. The current confinement structure and an optical waveguide structure due to loss guide can be realized by simply growing the n-type GaAs which forms the current confinement layer 6 and the p-type GaAs which forms the cap layer 7. Therefore, a satisfactory morphology can be obtained after the second and third growth processes because of the generally unrestricted range of conditions.
FIG. 2 shows a cross section of an essential part of a conventional semiconductor laser using the buried type structure. The semiconductor laser shown in FIG. 2 includes an n-type GaAs substrate 21, an n-type AlGaInP cladding layer 22, an InGaP active layer 23, a p-type AlGaInP cladding layer 24, a p-type InGaP buffer layer 25, a semiinsulative AlInP buried layer 26, a p-type GaAs cap layer 27, a p side electrode 28 and an n side electrode 29. In FIG. 2, W denotes a stripe width. This buried type structure is popularly used in the InP/InGaAsP system semiconductor lasers.
FIG. 3 shows a cross section of an essential part of a conventional semiconductor laser using the shaped substrate type structure. The semiconductor laser shown in FIG. 3 includes a p-type GaAs substrate 31, an n-type GaAs current confinement layer 32, a p-type InGaP buffer layer 33, a p-type AlGaInP cladding layer 34, an InGaP active layer 35, an n-type AlGaInP cladding layer 36 and an n-type GaAs cap layer 37. Presently, this shaped substrate type structure has the most advantageous structure from the point of view of producing a large output.
Next, a description will be given of the problems encountered in the conventional semiconductor lasers using the ridge type, buried type and shaped substrate type structures.
The problem of the ridge type structure shown in FIG. 1 is that this structure employs the loss guide system. Special circumstances occurring in the case of the AlGaInP system semiconductor laser will now be described in detail.
AlGaInP is a 4-element mixed crystal having a thermal resistance which is three times that of the AlGaAs system and over seventeen times that of the InP system. Hence, care should be taken when using AlGaInP so that a saturation does not occur due to heat. In order to suppress the generation of heat so as to prevent the saturation due to the heat, the length of the cavity is normally made long to reduce the density of the current flowing to the active layer. In addition, the small gain is increased by use of the long cavity so that it is possible to obtain a total gain required for the laser oscillation.
However, the loss per unit length is large because this ridge type structure uses the loss guide system. For this reason, the differential quantum efficiency .eta..sub.d greatly deteriorates as the cavity length becomes longer as may be seen from the following formula, where .eta..sub.i denotes the internal quantum efficiency, .alpha. denotes the waveguide loss, L denotes the cavity length, R denotes the reflectivity of the laser edge facet and l.sub.n denotes the function describing natural logarithm. EQU .eta..sub.d =.eta..sub.i .times.[(1/L)l.sub.n (1/R)]/[.alpha.+(1/L)l.sub.n (1/R)]
In other words, when the waveguide loss .alpha. is large and the cavity length L is large, it can be readily seen from the above formula that the differential quantum efficiency .eta..sub.d rapidly approaches zero.
Accordingly, the quantum efficiency of the loss guide system is poor, and it is extremely difficult to produce an output of 50 mW or greater while making the mode control by the loss guide. In addition, the light absorption at the n-type GaAs current confinement layer 6 is fed back as heat as the output becomes larger, thereby heating the semiconductor laser itself and making it difficult to produce a large output.
In order to overcome the above described problems of the ridge type structure, it is conceivable to carry out both the mode control and the current confinement by forming the current confinement layer 6 from an n-type AlInP which includes a quantity of Al greater than that included in the p-type AlGaInP cladding layer 4. In this case, however, it becomes necessary to form a layer which includes Al on a layer which includes Al. Normally, the morphology and characteristic of an AlInP layer is greatly affected by the surface on which the AlInP layer is formed, but generally, Al oxides tend to remain on the surface of the layer which includes Al. Hence, it is difficult to form the AlInP layer on the layer which includes Al.
Therefore, it is extremely difficult to produce a large output from the semiconductor laser using the ridge type structure.
On the other hand, various problems occur when the buried type structure shown in FIG. 2 is applied to the AlGaInP system semiconductor laser which emits a laser beam having a wavelength in the order of 0.6 .mu.m. First, the stripe width W which guarantees the zeroth transverse mode is narrow. Normally, in the semiconductor laser which emits a laser beam having a wavelength in the order of 1.5 .mu.m, the stripe width W with which the transverse mode becomes single is 2 .mu.m or less, but the stripe width W becomes approximately 1/3 or 1 .mu.m or less in the case of the semiconductor laser which emits a laser beam having a wavelength in the order of 0.6 .mu.m. Accordingly, it becomes necessary to carry out a mesa etching by setting the stripe width W to 1 .mu.m or less. Such a mesa etching may be realized using a reactive ion etching (RIE). But when the RIE is used, the crystal surface is damaged and it becomes impossible to satisfactory grow a layer on the damaged crystal surface. For this reason, the mesa etching must be made by a wet etching, but it is virtually impossible to accurately carry out the mesa etching to form the stripe width W of 1 .mu.m or less with a satisfactory reproducibility.
In addition, the semiinsulative AlInP buried layer 26 must be formed on the layer which includes Al, similarly as in the case of the ridge type structure, and it is impossible to satisfactorily form the semiinsulative AlInP buried layer 26.
Furthermore, in the AlGaInP system semiconductor laser, the energy band gap of the active layer is large and the voltage applied above and below the active layer is high during the operation. For example, the energy band gap of the InGaP active layer 23 is 1.85 eV or greater. Generally, the buried type structure suffers from a problem in that a leak current flows via the interface state at the interface between the mesa etched surface and the buried layer, and the rise of the operation voltage directly leads to the increase of the leak current. Moreover, since the buried layer 26 is formed on the layer which includes Al, the interface state exists at the interface between the mesa etched surface and the buried layer 26, making it difficult to reduce the leak current, and causes the decrease of the differential quantum efficiency .eta..sub.d. In addition, because the stripe width W is made narrow for the transverse mode control, the light energy density increases when producing the high output, and there is a problem in that a catastrophical optical damage (COD) breakdown easily occurs.
For the above described reasons, the buried type structure also has a large number of problems to be solved in order to produce a large output, similarly as in the case of the ridge type structure.
Next, the shaped substrate type structure shown in FIG. 3 will be studied. The shaped substrate type structure does not use the loss guide system, but uses the waveguide structure of the index guide by bending the active layer so that a small loss is realized. In addition, there is no need to form a layer which includes Al on a layer which includes Al. Hence, when the peculiarity of the AlGaInP system material is considered, the shaped substrate type structure may be best suited for producing a large output.
A description will be given of a method of producing the conventional semiconductor laser using the shaped substrate type structure. Normally, the shape of the substrate surface is formed by one of two methods or a combination thereof. In other words, an etching, a selective growth, or a combination of etching and selective growth may be used to form the shape of the substrate surface.
FIG. 4 shows a cross section of an essential part of the semiconductor laser using the shaped substrate type structure produced by the etching. The semiconductor laser shown in FIG. 4 includes an n-type GaAs substrate 41, a p-type GaAs current confinement layer 42, an n-type AlGaInP cladding layer 43, an InGaP active layer 44, a p-type AlGaInP cladding layer 45, a p-type InGaP buffer layer 46, a p-type GaAs contact layer 47, a guided light pattern 48 and an absorbing part 49. W denotes the stripe width, 0 denotes the inclination angle of the current confinement layer 42, and d denotes the thickness of the cladding layer 43.
When producing the semiconductor laser shown in FIG. 4, the p-type GaAs current confinement layer 42 is formed on the n-type GaAs substrate 41, and an etching is made in a &lt;111&gt; direction in which a conventional mesa can be formed using a H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 +H.sub.2 O system etchant. Thereafter, the n-type AlGaInP cladding layer 43, the InGaP active layer 44, the p-type AlGaInP cladding layer 45, the p-type InGaP buffer layer 46 and the p-type GaAs contact layer 47 are successively formed on the conventional mesa structure.
However, the following problems occur.
First, when etching the current confinement layer 42, the inclination angle .theta. becomes approximately 50.degree. and large when the normal etchant is used. Hence, the active layer 44 which is formed on above the current confinement layer 42 having the large inclination angle .theta. makes a large curve. As a result, the difference in the transverse refractive indexes which affects the laser beam becomes considerably large, and the stripe width W which can maintain the single transverse mode becomes 1 .mu.m or less. Consequently, problems similar to those encountered in the buried type structure are generated.
Second, when the thickness d of the n-type AlGaInP cladding layer 43 is made large in order to prevent the guided light from being absorbed within the p-type GaAs current confinement layer 42, the thermal resistance increases considerably and it becomes impossible to obtain a continuous wave (CW) oscillation due to the peculiarity of the AlGaInP system material that the thermal resistance thereof is considerably large.
Therefore, it was confirmed that it is extremely difficult to produce the semiconductor laser having the shaped substrate type structure by the etching.
FIGS. 5 and 6 respectively show cross sections of an essential part of the semiconductor laser using the shaped substrate type structure at essential stages of the selective growth production process.
As methods of realizing the selective growth, it is possible to use the LPE, MOVPE, molecular beam epitaxy (MBE) and the like. However, it is better to form the AlGaInP system material by the MOVPE because additional equipment is required for the mass production when a method other than the MOVPE is used to form the shape of the shaped substrate and also because the MOVPE is best suited for growing a uniform layer on a large area. Hence, the MOVPE was used to form a layer of the AlGaInP system material.
It is desirable to use GaAs or AlGaAs as the material for forming the desired shape because these materials are lattice matched to GaAs and the thermal resistance of these materials is small compared to that of the AlGaInP system material.
Generally, when selectively growing a GaAs or AlGaAs layer by the MOVPE or growing the GaAs or AlGaAs layer on the shaped substrate, the following facts have been confirmed. For the sake of convenience, it is assumed that the material used is Al.sub.x Ga.sub.1-x As, where x.ltoreq.0.3, and is substantially GaAs.
GaAs has a characteristic such that the growth on the surface having (111)B as the index of crystal face is extremely slow. Accordingly, when a GaAs layer is grown in a state where the GaAs substrate is masked in the &lt;110&gt; direction, the GaAs layer grows on the (111)A face of the conventional mesa but does not grow on the (111)B face of the re-entrant mesa.
FIG. 5 shows the above selective growth of the GaAs layer. The semiconductor laser includes a GaAs substrate 51, a SiO.sub.2 layer 52 and a GaAs layer 53. As shown, the cross section of the GaAs layer 53 becomes hexagonal by the selective growth.
But when the GaAs layer 53 is grown on the GaAs substrate 51 which is masked in the &lt;110&gt; direction, the (111)B face forms the conventional mesa. Hence, the cross section of the GaAs layer 53 becomes trapezoidal, and the growth stops when a vertex part of a triangular cross section is formed.
FIG. 6 shows the formation of the GaAs layer 53 having the triangular cross section. In FIG. 6, those parts which are the same as those corresponding parts in FIG. 5 are designated by the same reference numerals, and a description thereof will be omitted.
The growth characteristics explained in conjunction with FIGS. 5 and 6 also holds true when a semiconductor layer is formed on the shaped substrate having the mesa structure. For example, when growing the semiconductor layer on the conventional mesa stripe which extends in the &lt;110&gt; direction, the (111)A face appears at the sloping surface of the mesa and the semiconductor layer is grown on the conventional mesa stripe including the (111)A face. When growing the semiconductor layer on the conventional mesa stripe which extends in the &lt;110&gt; direction, the mesa stripe has a triangular cross section at the central part and the semiconductor layer having a gradual sloping surface which is the (311)B face grows from the flat surface and crawls on the mesa side surface.
FIG. 7 shows a cross section of a structure for explaining the growth of the semiconductor layer on the conventional mesa which is formed by the etching and extends in the &lt;110&gt; direction. The structure shown in FIG. 7 includes a substrate 61, a mesa stripe 61A which extends in the &lt;110&gt; direction, a first semiconductor layer 62 and a second semiconductor layer 63.
FIG. 8 shows a cross section of a structure for explaining the growth of the semiconductor layer on the conventional mesa which is formed by the etching and extends in the &lt;110&gt; direction. In FIG. 8, those parts which are the same as those corresponding parts in FIG. 7 are designated by the same reference numerals, and a description thereof will be omitted. In this case, the surface of the semiconductor layer 63 which is formed on the mesa stripe 61A has a index of crystal face (100). the mesa stripe
For example, a sloping surface having an extremely small angle with respect to the substrate surface as shown in FIG. 8 is extremely difficult to form by the etching. However, if a mesa stripe having such a gradual sloping surface can be formed and applied to the semiconductor laser having the shaped substrate type structure, it would be possible to set the stripe width W for maintaining the single transverse mode to a large value and thereby enable a large output to be produced.
FIG. 9 shows a cross section of an essential part of a known GaAs/AlGaAs system semiconductor laser which is produced using the shaped substrate having the gradual sloping surface. The semiconductor laser includes a p-type GaAs substrate 61, a p-type GaAs buffer layer 62, a p-type AlGaAs buffer layer 63, an n-type GaAs current confinement layer 64, a p-type AlGaAs cladding layer 65, an AlGaAs active layer 66, an n-type AlGaAs cladding layer 67, an n-type GaAs contact layer 68, an n side metal electrode 69 and a p side metal electrode 70. W denotes the stripe width required to maintain the single transverse mode, .theta. denotes the angle formed by the (100) face and a line which connects an intersection point of the (100) face and the (111)B face and a line which defines the stripe width W.
According to this semiconductor laser, the layers up to the contact layer 68 are continuously grown on the substrate 61 indicated by a broken line, and the transverse mode control is made by use of the bend where the (100) face and the (311)B face of the active layer 66 meet.
But when the method used to produce the semiconductor laser shown in FIG. 9 is used as it is to produce the AlGaInP system semiconductor laser, the following problems occur.
That is, as may be seen from FIG. 9, the flat part of the active layer 66 spreads with reference to the vertex where the (111)B faces of the substrate 61 meet. For this reason, in order to prevent the COD breakdown even when the large output is produced, it is necessary to make the cladding layer 65 under the active layer 66 thick. But in the AlGaInP system semiconductor laser, the cladding layer 65 is of course made of AlGaInP which is a 4-element material having a large thermal resistance as described above. Therefore, the thermal resistance would become even larger if the thickness of the cladding layer 65 is increased, and the CW oscillation would become impossible.
On the other hand, other types of semiconductor lasers have been proposed in Japanese Laid-Open Patent Applications No. 55-158689, No. 64-30287 and the like.
FIG. 10 shows a semiconductor laser proposed in the Japanese Laid-Open Patent Application No. 55-158689. This semiconductor layer includes an n-type GaAs substrate 920, a p-type Ga.sub.1-x Al.sub.x As current blocking layer 921, an n-type Ga.sub.1-x Al.sub.x As cladding layer 922, an n or p-type GaAs active layer 923, a p-type Ga.sub.1-x Al.sub.x As cladding layer 924, a p-type GaAs ohmic contact layer 925, a p mode electrode 926, an n side electrode 927, a triangular prism shaped mesa part 928, a damaged part 929 and a light emitting region 930.
However, according to this structure, since the angle .theta. of the mesa part 928 is large and the mesa part 928 is formed by the LPE, the active layer 923 formed thereon greatly curves. For this reason, the stripe width which in effect determines the waveguide is restricted by the bent part of the active layer 923, and it is extremely difficult to form the active layer 923 to a width of 2 .mu.m or greater in this structure. Consequently, the degree of freedom with which the stripe width can be controlled is extremely small.
On the other hand, FIG. 11 shows shows a semiconductor laser proposed in the Japanese Laid-Open Patent Application No. 64-30287. This semiconductor layer includes a p-type GaAs substrate 901, a stripe convex part 902, a p-type GaAs buffer layer 903, p-type AlGaAs cladding layers 904 and 906, a current blocking layer 905, an AlGaAs active layer 907, an n-type AlGaAs cladding layer 908, an n-type GaAs contact layer 909, an n side ohmic electrode 910 and a p side ohmic electrode 911.
This structure basically employs the same principle as the structure shown in FIG. 9, and the stripe width and the thickness of the cladding layer cannot be selected independently. This structure differs from that shown in FIG. 9, however, in that the stripe convex part 902 formed on the substrate 901 has a re-entrant mesa shape. The cladding layer 904 which is formed on the substreate 901 grows on the side surface of the re-entrant mesa of the stripe convex part 902 and on the (111)B face at the side surface of the conventional mesa at the upper part of the stripe convex part 902. However, when the side surface of the re-entrant mesa and the side surface of the conventional mesa in the stripe convex part 902 have completely different indexes of crystal face, it was found from the experiments conducted by the present inventors that the morphology of the cladding layer 904 at such side surfaces becomes extremely poor. The present inventors have also found from other experiments that the morphology of a layer is satisfactory when the layer is grown on a side surface which is made up of only the (111)B face on which the layer growth is slow. For the above described reasons, it is difficult to produce a smooth stripe structure in the semiconductor laser proposed in the Japanese Laid-Open Patent Application No. 64-30287. Furthermore, since the scattering loss is large, it is impossible to produce a large output from this semiconductor laser. It was also found from the experiments conducted by the present inventors that the side surface of the mesa is desirably made up solely of the (111)B face.
For the foregoing reasons, there is a problem in that the semiconductor lasers shown in FIGS. 10 and 11 are also unsuited for producing a large output.
Another semiconductor laser having the mesa structure is also proposed in a Japanese Laid-Open Patent Application No. 64-32692. However, illustration and description thereof will be omitted in this specification because the proposed structure is basically similar to the structure shown in FIG. 11.